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Genitalia are among the fastest evolving morphological traits as evidenced by their
common function as diagnostic traits in species identification. Even though the main
function of genitalia is the successful transfer of spermatozoa, the presence of diverse
structures that are obviously not necessary for this suggests that genitalia are a
target of sexual selection. The male genitalia of many spider species are extremely
complex and equipped with numerous sclerites, plates and spines whose functions are
largely unknown. Selection on male genitalia may be particularly strong in sexually
cannibalistic spiders, where mating success of males is restricted to a single female.
We investigated the copulatory mechanism of the sexually cannibalistic orb weaving
spider Argiope bruennichi by shock freezing mating pairs and revealed a complicated
interaction between the appendices and sclerites that make up the male gonopods (paired
pedipalps). The plate that covers the female genital opening (scape) is secured between
two appendices of the male genital bulb, while three sclerites that bear the sperm
duct are unfolded and extended into the female copulatory opening. During copulation,
females attack and cannibalise the male and males mutilate their genitalia in about
80% of cases. Our study demonstrates that (i) genital coupling is largely accomplished
on the external part of the female genitalia, (ii) that the mechanism requires an
interaction between several non-sperm-transferring structures and (iii) that there
are two predetermined breaking points in the male genitalia. Further comparative work
on the genus Argiope will test if the copulatory mechanism with genital mutilation
indeed is an adaptation to sexual cannibalism or if cannibalism is a female counter
adaptation to male monopolisation through genital plugging.